Since the discovery of mechanically exfoliated graphene in 2004, research on ultrathin two-dimensional (2D) nanomaterials has grown exponentially in the fields of condensed matter physics, material science, chemistry, and nanotechnology. Highlighting their compelling physical, chemical, electronic, and optical properties, as well as their various potential applications, in this Review, we summarize the state-of-art progress on the ultrathin 2D nanomaterials with a particular emphasis on their recent advances. First, we introduce the unique advances on ultrathin 2D nanomaterials, followed by the description of their composition and crystal structures. The assortments of their synthetic methods are then summarized, including insights on their advantages and limitations, alongside some recommendations on suitable characterization techniques. We also discuss in detail the utilization of these ultrathin 2D nanomaterials for wide ranges of potential applications among the electronics/optoelectronics, electrocat...

Recent Advances in Ultrathin Two-Dimensional Nanomaterials

High-performance gas sensors prepared using p-type oxide semiconductors such as NiO, CuO, Cr2O3, Co3O4, and Mn3O4 were reviewed. The ionized adsorption of oxygen on p-type oxide semiconductors leads to the formation of hole-accumulation layers (HALs), and conduction occurs mainly along the near-surface HAL. Thus, the chemoresistive variations of undoped p-type oxide semiconductors are lower than those induced at the electron-depletion layers of n-type oxide semiconductors. However, highly sensitive and selective p-type oxide-semiconductor-based gas sensors can be designed either by controlling the carrier concentration through aliovalent doping or by promoting the sensing reaction of a specific gas through doping/loading the sensor material with oxide or noble metal catalysts. The junction between p- and n-type oxide semiconductors fabricated with different contact configurations can provide new strategies for designing gas sensors. p-Type oxide semiconductors with distinctive surface reactivity and oxygen adsorption are also advantageous for enhancing gas selectivity, decreasing the humidity dependence of sensor signals to negligible levels, and improving recovery speed. Accordingly, p-type oxide semiconductors are excellent materials not only for fabricating highly sensitive and selective gas sensors but also valuable additives that provide new functionality in gas sensors, which will enable the development of high-performance gas sensors.

Highly sensitive and selective gas sensors using p-type oxide semiconductors: Overview

Energy shortage, environmental crisis, and developing customer demands have driven people to find facile, low-cost, environmentally friendly, and nontoxic routes to produce novel functional materials that can be commercialized in the near future. Amongst various techniques, the hydrothermal carbonization (HTC) process of biomass (either of isolated carbohydrates or crude plants) is a promising candidate for the synthesis of novel carbon-based materials with a wide variety of potential applications. In this Review, we will discuss various synthetic routes towards such novel carbon-based materials or composites via the HTC process of biomass. Furthermore, factors that influence the carbonization process will be analyzed and the special chemical/physical properties of the final products will be discussed. Despite the lack of a clear mechanism, these novel carbonaceous materials have already shown promising applications in many fields such as carbon fixation, water purification, fuel cell catalysis, energy storage, CO(2) sequestration, bioimaging, drug delivery, and gas sensors. Some of the most promising examples will also be discussed here, demonstrating that the HTC process can rationally design a rich family of carbonaceous and hybrid functional carbon materials with important applications in a sustainable fashion.

Engineering Carbon Materials from the Hydrothermal Carbonization Process of Biomass

Metal oxide-based resistive-type gas sensors are solid-state devices which are widely used in a number of applications from health and safety to energy efficiency and emission control. Nanomaterials such as nanowires, nanorods, and nanoparticles have dominated the research focus in this field due to their large number of surface sites facilitating surface reactions. Previous studies have shown that incorporating two or more metal oxides to form a heterojunction interface can have drastic effects on gas sensor performance, especially the selectivity. Recently, these effects have been amplified by designing heterojunctions on the nano-scale. These designs have evolved from mixed commercial powders and bi-layer films to finely-tuned core–shell and hierarchical brush-like nanocomposites. This review details the various morphological classes currently available for nanostructured metal-oxide based heterojunctions and then presents the dominant electronic and chemical mechanisms that influence the performance of these materials as resistive-type gas sensors. Mechanisms explored include p–n and n–n potential barrier manipulation, n–p–n response type inversions, spill-over effects, synergistic catalytic behavior, and microstructure enhancement. Tables are presented summarizing these works specifically for SnO2, ZnO, TiO2, In2O3, Fe2O3, MoO3, Co3O4, and CdO-based nanocomposites. Recent developments are highlighted and likely future trends are explored.

Nanoscale metal oxide-based heterojunctions for gas sensing: A review

This review paper encompasses a detailed study of semiconductor metal oxide (SMO) gas sensors. It provides for a detailed comparison of SMO gas sensors with other gas sensors, especially for ammonia gas sensing. Different parameters which affect the performance (sensitivity, selectivity and stability) of SMO gas sensors are discussed here under. This paper also gives an insight about the dopant or impurity induced variations in the SMO materials used for gas sensing. It is concluded that dopants enhance the properties of SMOs for gas sensing applications by changing their microstructure and morphology, activation energy, electronic structure or band gap of the metal oxides. In some cases, dopants create defects in SMOs by generating oxygen vacancy or by forming solid solutions. These defects enhance the gas sensing properties. Different nanostructures (nanowires, nanotubes, heterojunctions), other than nanopowders have also been studied in this review. At the end, examples of SMOs are given to illustrate the potential use of different SMO materials for gas sensing.

Semiconductor metal oxide gas sensors: A review

In order to sensitively detect indoor air contaminants including benzene, formaldehyde, toluene, and acetone, an effective method using a gas sensor based on coral-shaped tin dioxide nanostructures is reported. It is found that the presented gas sensor exhibits obviously enhanced sensing performance toward indoor air contaminants compared with the ones based on some conventional nanostructures, like nanospheres and nanoparticle-based thin film. The mechanism for such fascinating improvement is demonstrated from the nanoscale size-dependent effect by following theoretical models. Our findings not only provide a promising environmental sensor for sensitively detecting indoor air contaminants, but also suggest a general strategy for utilizing the nanoscale effects of nanoparticles in other environmental and energy-related applications, like waste-water absorbents, catalysts, and solar cells.

Sensitive detection of indoor air contaminants using a novel gas sensor based on coral-shaped tin dioxide nanostructures

The invention discloses a foam metal-based oil slick collection material and the preparation method thereof. The foam metal-based oil slick collection material is formed by embedding foam metal in a hydrophobic and hydrophilic material, drying and curing. The foam metal based oil slick collection material uses the metal foam as a matrix material and the hydrophobic and hydrophilic material is modified, so that the mechanical strength and oil slick adsorption capability of the oil slick collection material are improved and the oil slick collection material can be adaptive to the dynamic oil slick collection process; the oil slick collection material can be used for petroleum leakage treatment, organic solvent leakage treatment and separation of industrial organic phase and water phase and also used for cleaning the oil slick on the surfaces of rivers, lakes and oceans; the oil slick collection material is particularly suitable for applications of ship-mounted water surface oil slick treatment devices.

Foam metal-based oil slick collection material and preparation method thereof

Although the relative response of metal oxide chemiresistive gas sensors has been greatly improved by using their nanostructures as sensing units, it remains challenging to recognize the detected gases due to their intrinsic cross-sensing properties. Here, by considering the temperature dependence of the sensing activity for metal oxide gas sensors, the dynamic sensing behavior of CdO (9 at%)-decorated porous ZnO nanobelts as an example was systematically investigated under a continuously changing working temperature. This working temperature modulation was achieved by precisely manipulating high and low heating voltages with a rectangular wave mode. In some working temperature ranges, the fabricated gas sensor exhibited characteristic response curves toward propanol, isopropanol, and ethanol, which were discriminated from other investigated volatile organic compounds (VOCs). Under periodical working temperature modulation, their characteristic response curves were repeatable and stable. In addition, the relationships between the relative responses at characteristic sensing temperature points and their concentrations were investigated. This work offers a promising strategy for using sensing nanomaterials for the highly sensitive recognition and analysis of VOCs, while avoiding cross-sensing.

A Temperature-Modulated Gas Sensor Based on CdO-Decorated Porous ZnO Nanobelts for the Recognizable Detection of Ethanol, Propanol, and Isopropanol

A microfiber knot ring (MKR) with a temperature sensitivity of 183 pm/℃ has been proposed and experimentally demonstrated. To further improve its stability and sensitivity, the polydimethylsiloxane (PDMS) film was used to encapsulate it to be a slice probe. The temperature sensitivity was increased for more than 9 folds with a good stability and repeatability. The contribution ratios of thermal-optic coefficient and thermal-expansion coefficient of silica and PDMS were analyzed and discussed. The high sensitivity of 1.67 nm/℃ and miniature slice structure enables its application for precisely monitoring the temperature fluctuation in some special occasions.

https://iopscience.iop.org/article/10.1088/1402-4896/ab34ec/pdf

Fanli Meng

Papers

Sensitive detection of indoor air contaminants using a novel gas sensor based on coral-shaped tin dioxide nanostructures

Foam metal-based oil slick collection material and preparation method thereof

A Temperature-Modulated Gas Sensor Based on CdO-Decorated Porous ZnO Nanobelts for the Recognizable Detection of Ethanol, Propanol, and Isopropanol

Temperature measurement using a microfiber knot ring encapsulated in PDMS

Microscale analysis and gas sensing characteristics based on SnO2 hollow spheres